1. Field of the Invention
The present invention relates in general to a printing apparatus, more specifically, to a sheet discharge device for discharging a print sheet toward a discharge tray.
2. Description of the Related Arts
In general, a printing apparatus has a sheet feed section to send out a top print sheet of a stack of paper loaded on a sheet feed tray, a printing section to carry out printing on the print sheet supplied from the sheet feed section, a sheet discharge device to discharge a print sheet, and a discharge tray to receive the print sheet discharged by the sheet discharge device.
Moreover, it takes some time for wet ink on the print sheet to dry. Therefore, when sheets of printing paper are printed successively, a print sheet being discharged now may brush a printed surface of a previously print sheet that is stacked on the top of the discharge tray, thereby smearing a printed image thereon.
Thus, in order to stack up print sheets on the discharge tray, it is desired to make a print sheet fall at right angles to the print sheet(s) on the discharge tray.
For instance, Japanese Unexamined Patent Application Publication No. 2001-240290 discloses a sheet discharge device includes a pair of flying induction members that function as jump wings for flying a print sheet having been carried by a conveyor belt (or transfer belt), while maintaining the print sheet in a bent form (roughly U shape).
In order to give a roughly U shape curl in the sheet, the flying induction member pair shift in the widthwise direction of the sheet according to information (size of the sheet) of a print sheet, and the two opposite faces come in contact with two side edges in the widthwise direction of the print sheet. Accordingly, a print sheet being discharged maintains a horizontal state with respect to a sheet transfer direction, and ends of the discharged sheets are aligned neatly.
However, the thickness of a print sheet is expressed in terms of weight per unit area (g/m2). For example, a print sheet is described as ‘regular’ if the thickness falls within the range of 58 g/m2 to 105 g/m2, ‘thick’ if the thickness is greater than the range, and ‘thin’ if the thickness is less than the range. A typical sheet discharge device determines the distance between sheet discharge wings (flying induction members) corresponding to a regular print sheet of various sizes.
What happens in the case of discharging a thick print sheet more elastic than a regular sheet is that, when an attempt is made to give a rough U shape curl to the print sheet (thick sheet), the repulsive elasticity of the thick sheet wins over a nip force (the dead weight of a sheet discharge roller, or its dead weight+spring pressure) and a suction force of a suction section maintaining a print sheet on a conveyor belt, so the thick sheet is not curved in a roughly U shape but floats instead. As a result, the print sheet may not be conveyed. This problem occurs more easily as the repulsive elasticity of a sheet increases even more toward a bottom of the sheet in a cantilever state where the dead weight for pulling the sheet down is greater. If the nip force and the suction force are increased to keep a print sheet from floating, however, the sheet may be damaged during conveying, or problems like ink transfer contamination, a blurred looking image etc., may occur.
Therefore, when thick sheets are used for printing in a conventional printer, it is necessary to broaden the width of a sheet discharge wing (or lower the installation height of the sheet discharge wing) so as to give a small U shape curl to the sheet. However, because a sheet width during discharge increases correspondingly to a weak curl of a print sheet into the U shape, the width of a side fence of a V-shaped discharge tray shown in FIG. 2 has to be broadened greater than the width of loaded sheets. In consequence, a stack of loaded print sheets on the discharge tray may not be evenly arranged in the widthwise direction, thereby impairing alignment accuracy of sheet discharge.
In contrast, thin sheets are much less elastic than the regular sheet. Therefore, a conventional sheet discharge device sometimes fails to maintain a thin print sheet in a U shape during discharge, and the thin print sheet could sag downward from an exit port due to its dead weight. If the U shape curl of such print sheet is made in large scale to prevent any sagging of the print sheet, the width of a print sheet falling down onto the discharge tray becomes extremely narrower than the width defined by the side fences of the discharge tray. Hence, the print sheet cannot maintain its U shape and alignment of discharged sheets is not stable and uniform. Moreover, with a conventional sheet discharge device, a print sheet falling on the discharge tray comes in contact with the side fence thereof and the falling speed of the print sheet decreases by that. However, in the case of thin print sheets, no contact is made between the thin print sheets and the side fences of the discharge tray, so the falling speed of the sheets are not decreased. Rather, the thin print sheets may fly up while falling due to air resistance and jump over the end fence of the discharge tray.
Especially, the problems with thin sheets are more evident in ink jet printing or double-sided printing where a sheet discharge speed is set to lower than offset printing because print sheets at the exit port stay longer in the cantilever state.